A combined cycle power plant may include a gas turbine that generates power output from combustion of a fuel and air mixture. A heat recovery steam generator may be located downstream from the gas turbines to receive exhaust gas from the gas turbines. The heat recovery steam generator extracts energy from the exhaust gas to produce superheated steam. The superheated steam may be transferred to a steam turbine for generating additional power output. A steam turbine may include multiple pressure steam sections such as a high pressure steam section, an intermediate pressure steam section, and one or more low pressure steam section. The combined cycle power plant may be in a single-shaft application. A single-shaft application may include one generator that is connected to the gas turbine and the steam turbine. The combined cycle power plant may be in a multiple-shaft application. A multiple-shaft application may include multiple generators. Each of the multiple generators may be connected to the gas turbine and the steam turbine separately.
It is often desired and required to operate the combined cycle power plant at a load below the maximum rating of the plant. This scenario is termed as part load operation. It is also known that the thermal efficiency of combined cycle Bratyon Cycle and Rankine Cycle engines is significantly reduced when operating at part loads. For example, a steam turbine in the Rankine cycle being a constant volume system would operate at reduced throttle pressure when operating at lower loads, i.e., a lower steam flow. Consequently, this operation may reduce the theoretical maximum reversible operation for steam expansion and will reduce the power output. Therefore, a need remains to improve performance of a combined cycle power plant at lower loads.